1. Field of the Invention
The present invention relates to a polymer solid electrolyte and a lithium secondary cell adopting the same. More particularly, the invention relates to a polymer solid electrolyte having excellent ion conductivity which can be processed easily due to its excellent mechanical property and a lithium secondary cell adopting the same.
2. Description of Related Art
Secondary cells having high energy density, used as a driving source thereof, are required as portable electronic appliances such as camcorders, cellular phones, laptops, etc. become smaller and lighter. There are ten or more types of secondary cells including the nickel-cadmium cell, the nickel-hydrogen cell, the lithium ion cell, and the like. In particular, the lithium ion cell has been the focus as a next generation power source due to its long lifetime and high capacity.
Research into the lithium secondary cell began in the early 1970s, and there has been competition among worldwide research institutes to develop a lithium secondary cell for practical use. Sony Energy Tech. Co. developed a lithiumcarbon secondary cell having a lithium anode obtained from lithium-cobalt oxide and a carbon cathode. Also, Moly Energy Co. commercialized a lithium-carbon secondary cell having a lithium anode obtained from lithium nickel oxide and a carbon cathode.
Lithium compounds that are typically used as an cathode active material for lithium secondary cells include lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2) or lithium manganese oxide (LiMn.sub.2 O.sub.4). Lithium compounds that are typically employed as a anode active material include metallic lithium, lithium alloy or carbon. In addition, liquid or solid electrolyte can be used as an electrolyte. If a liquid electrolyte is used, however, many problems can occur which are related to stability, e.g., breakage of the cell caused by evaporation, and the like. In order to solve these problems, some have proposed to use solid electrolyte instead of the liquid electrolyte.
Solid electrolytes generally do not leak electrolyte solution and are easily processed. Thus, research into the solid electrolyte has been actively performed. There is a particularly high interest in a polymer solid electrolyte. Polymer solid electrolytes can be classified into: (i) a solid type without an organic electrolyte solution; and (ii) a gel type including organic electrolyte solution.
A crosslinked polyether substance is included in the solid type. The solid type electrolyte usually has a conductivity of about 10.sup.-4 S/cm. However, in order to apply the electrolyte to a cell, a conductivity of 10.sup.-3 S/cm or more is required. Thus, it is difficult to practically use the solid type electrolyte due to its insufficient conductivity and poor mechanical property.
On the other hand, the gel type solid electrolyte is of great interest and focus as a useful material of a secondary cell due to its conductivity of 10.sup.-3 S/cm or more and its sufficient mechanical intensity. For example, Japanese Laid-open Patent Publication No. 4-306560 discloses a polymer solid electrolyte including a copolymer of acrylonitrile and methyl acrylate or a copolymer of acrylonitrile and methyl methacrylate, support electrolyte salt and solvent such as propylene carbonate.
This kind of electrolyte however, has the following problems. First, the selection range of solvent used for manufacturing the electrolyte is very restricted. Second, the manufacturing of the electrolyte is difficult and high-temperature conditions of above 180.degree. C. are required. Third, due to its poor ability of holding organic electrolyte solution (including the support electrolyte salt and solvent), the organic electrolyte solution may leak during the usage of cell.
As another example of the gel type solid electrolyte, Japanese Laid-open Patent Publication No. 3-207752 discloses a polymer solid electrolyte manufactured by irradiating ultraviolet rays onto a liquid composition including polyethylene glycol and/or dimethacrylate, support electrolyte salt and solvent. The electrolyte has excellent plasticity and poor conductivity of 10.sup.-4 S/cm or less, so that it is difficult to apply the electrolyte to a cell.
To solve the above problems by increasing conductivity, U.S. Pat. No. 5,463,179 introduces a rigid functional group such as an alkyl group in a polymer matrix, resulting in a stable three-dimensional space within the polymer matrix. The processing property thereof, however, is not suitable even if the conductivity is remarkably increased.